Acrylic acid is currently manufactured by the vapor phase catalytic oxidation of propylene or acrolein. The gaseous reactor effluent from this process contains from about 10 to 80 wt.% acrylic acid, with water, acetic acid and various organic impurities. This stream is then condensed or absorbed to obtain an aqueous solution of acrylic acid, acetic acid and water.
Purification of this stream to obtain acrylic acid has been extremely difficult. The relative volatilities of acrylic acid, acetic acid and water prevents simple fractional distillation as a method of purification.
Several processes have been proposed for this purification. U.S. Pat. No. 3,816,524 describes a method of separating acrylic acid by using various solvents to extract the acrylic acid from the aqueous solution. U.S. Pat. No. 3,432,401 describes a method of using specific solvents as entrainers to aid in azeotropic distillation.
Both of these prior art processes suffer various disadvantages in that the temperatures necessary to perform distillation and solvent recovery can cause polymerization of the acid. Further, in extraction systems large amounts of solvent are necessary and such systems tend to have a high solvent loss.
As stated in U.S. Pat. No. 3,432,401, fractional crystallization has not been considered as a means for recovering acrylic acid because a eutectic between acrylic acid and water exists at 63% by volume of acrylic acid. Below the eutectic point, the acrylic acid concentrates in a liquid phase, while above the eutectic, the acid concentrates in a solid phase. The existence of the eutectic precludes complete separation of acrylic acid and water by direct fractional crystallization.
Salts have been used in the process for the recovery of acrylic acid for various purposes. U.S. Pat. No. 2,922,815 discloses that calcium chloride, sodium sulfate, and dry metal salts such as nickel chloride and nickel bromide have been used as drying agents to concentrate the acrylic acid found in the aqueous solution up to about 80%. U.S. Pat. No. 3,846,488 discloses a method for accelerating the separation of an organic solvent phase from an aqueous phase obtained by the extraction of acrylic acid with a solvent by adding an alkali metal salt or ammonium salt in minute amounts to the aqueous solution.
Rather than accelerating separation between a solvent and water, or drying the aqueous solution, it has been discovered that the addition of certain salts to the aqueous solution removes the eutectic point between acrylic acid and water thereby allowing simple fractional crystallization.